识别终止密码子

Question

I was trying to write some code that identifies stop codons in my sequence. I wrote the following code, but when I try it out, it stops after I type in my sequence when prompted. I am not sure why it doesn't go through. Thanks a lot!

dna=input("Please type your DNA sequence: ")

stopcodons=("TAG","TGA","TAA")
pos=0

while (pos+2)<(len(dna)):
    if dna[pos:pos+3] in stopcodons:
        type("Your sequence has a stop codon at %s position." %pos)
    else:
        pos=pos+3

Answer 1

your loop is never going to end if you find a position because you do not increment pos. Also, I'm not sure why you increment by 3 pos.

while (pos+2)<(len(dna)):
    if dna[pos:pos+3] in stopcodons:
        type("Your sequence has a stop codon at %s position." %pos)
    pos=pos+1

should work better

Answer 2

The reason why your program goes into an infinite loop once it has found a stop codon, is because you instruct your program to only increment pos (and thus move the cursor ) in the else case : when you did not found a sequence.

You thus have to update the cursor every time:

while (pos+2)<(len(dna)):
    if dna[pos:pos+3] in stopcodons:
        "Your sequence has a stop codon at %s position." %pos)
    

(or even better: use pos += 3 instead of pos = pos + 3 ).

You also have to use print instead of type since type simply obtains the type of an object.

Note that it is usually safer to use a range(..) in a for loop, like:

:
    if dna[pos:pos+3] in stopcodons:
        print("Your sequence has a stop codon at %s position." %pos)

Answer 3

You almost NEVER want to use a counter, or index into arrays/strings directly in Python.

dna=input("Please type your DNA sequence: ")

stopcodons=("TAG","TGA","TAA")

for stopcodon in stopcodons:
    if stopcodon in dna:
        print("Your sequence has a stop codon at %s position" % dna.find(stopcodon))

The above is more "Pythonic" and helps to prevent numerous mistakes that are all too common in less advanced languages.